You can see and feel the difference immediately.
Purified brazing flux is black brazing flux that has had extra processing steps. These processing steps take the black article out and leave the flux a rich, creamy brown color. If you take a little of each flux and rub it between your fingers you can feel that Purified Flux is not only smoother but the particles are smaller and there are no extra large particles.
Brazing Flux is made to prevent oxygen from getting to the parts as they are heated. Steel and especially tungsten carbide oxidize at room temperature. The hotter they get the more they oxidize. Above 1,000 F tungsten carbide oxidizes extremely rapidly and forms an unbrazeable surface. Purified flux is good for an more time at higher temperatures, up to 1,700 F.
The original idea with flux was that it was to be applied on top of the braze area. No special effort was made to purify the flux because it worked well and they wanted to keep the cost down.
However the critical part of saw and tool brazing is what goes on inside the braze joint. Ordinary flux is inexpensively made and has up to 10% odd size particles and non-active minerals in it. These odd size particles and non-active minerals get lodged in the braze area and can seriously effect the strength of the braze joint.
Purified flux is cleaner, smoother, creamier and much more effective.